Rotational molding, more commonly known as rotomolding, is generally used for molding hollow articles such as toys, sports equipment, fuel tanks, playground equipment, tanks for use in agriculture, refuse containers, and large chemical tanks. Many of these products are left out-of-doors, unprotected against the weathering elements of rain, and of heat and ultraviolet light from the sun.
To rotomold a part, powdered polyethylene or other plastic resin is placed inside a hollow mold, which is then closed and rotated on two axes, inside a heated oven to allow the powder to fuse together, forming a solid, hollow mass on the inside of the mold. The mold is then removed from the oven, to cool by air or water spray. After cooling, the molded part is removed, and more powder is placed inside the mold to begin another cycle. A more detailed discussion of rotomolding may be found in Modern Plastics Encyclopedia 1990, pages 317-318.
Polymers of ethylene, including homopolymers and copolymers, are often used in the rotomolding process. Into these polymers are frequently incorporated thermal stabilizers, ultraviolet absorbers and crosslinking agents. Known thermal stabilizers include a combination of thioesters, disclosed in U.S. Pat. No. 2,956,982, with an organophosphite and a hindered phenolic antioxidant. Some combinations of an organophosphite and a hindered phenolic antioxidant are disclosed in U.S. Pat. No. 4,187,212. Known ultraviolet stabilizers include 2-hydroxy-4-n-octoxybenzophenone, and a hindered amine such as poly[(6-morpholino-s-triazine-2,4-diyl)[(2,2,6,6-tetramethyl-4-piperidyl)i mino]-hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino]] or bis(2,2,6,6-tetramethylpiperidine-4)sebacate, used separately or in combination.
Crosslinking agents may include a combination of organic peroxide initiator and a crosslinking co-agent. This combination is disclosed in U.S. Pat. No. 4,900,792 assigned to Allied-Signal and European Patent Application 87210 assigned to DuPont Canada. For rotomolding, dialkyl peroxides used are 2,5-dimethyl-2,5-di(t-butylperoxy)hexane or alpha,alpha'-bis(t-butylperoxy)diisopropylbenzene or those disclosed in U.S. Pat. No. 3,214,422. Of those disclosed in the patent, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane-3 is most commonly used. Co-agents used by those experienced in the art of crosslinking polyethylene, include triallyl cyanurate, triallyl isocyanurate, triallyl trimellitate, trimethololpropane trimethacrylate, and related monomers.
There continues to be a need for a crosslinkable polyethylene-based composition suited for rotomolding applications and characterized, when cured, by long term thermal stability and resistance to ultraviolet light-induced photodegradation.